Catalysts comprising cobalt on a support such as silica or alumina are known to be useful in hydrogenation reactions such as hydrogenating aldehydes and nitriles. They are also used in hydrocarbon synthesis via the Fischer-Tropsch process.
In the case of the Fischer-Tropsch hydrocarbon synthesis process, especially when conducted in a slurry bubble column reactor, one preferred catalyst comprises cobalt supported on a titania support. Generally, the titania in such catalysts is in its rutile form.
Promoter metals, such as rhenium, zirconium, manganese and Group VIII noble metals are commonly used with cobalt catalysts to improve various aspects of catalytic performance. For example, the presence of Re or a Group VIII metal has a beneficial effect on the dispersion of Co crystallites.
The activity of a cobalt-containing Fischer-Tropsch catalyst has been shown to be proportional to the surface cobalt sites for cobalt particle sizes above 6 nm. See Journal of Catalysis, 1992, 137(1), 212-224. It also has been shown that metal crystallites that have a poor nanoscale homogeneity tend to agglomerate to a more severe degree than those with homogeneous nanoscale distribution. See Stud. Surf Sci. and Catalysis, Vol. 162, 2006, pg 103-110. Purportedly, the optimum cobalt crystallite size for Fischer-Tropsch catalysis is in the range of 6 nm since this provides a larger number of surface cobalt sites than larger crystallites and since crystallites below 6 nm have lower site activity than those of 6 nm and larger. See den Breejen, et al., “On the Origin of the Cobalt Particle Size Effects in Fischer-Tropsch Catalysis”, Journal of American Chemical Society, (2009), 131(20), 7197-7203).
Thus, one object of the present invention is to prepare cobalt-containing catalysts that have an appropriate cobalt crystallite size and good nanoscale homogeneity.
Patent Publication WO 2007/071899 A1 discloses that exposing a silica-supported cobalt nitrate to a gas mixture containing NO and less than 5 vol % O2 in He at the nitrate decomposition temperature and then followed by reduction results in the formation of very small metal particles.
Another object of the present invention is to provide a method for forming a cobalt catalyst having small cobalt crystallite size with good nanoscale distribution without the use of NO and He (or NO with any inert or oxidizing gas).
In U.S. Pat. No. 4,568,663, there is disclosed a Fischer-Tropsch hydrocarbon synthesis process which utilizes a catalyst comprising cobalt, rhenium and titania. This catalyst is made by impregnating a titania support with an aqueous solution of cobalt nitrate and perrhenic acid by the conventional incipient wetness method, drying and then calcining to decompose the cobalt nitrate to the oxide. The presence of rhenium serves several important functions: one is to aid in the dispersion of the cobalt; another is to aid in the reduction of the cobalt oxide when the catalyst is activated (reduced). High dispersion and full reduction of the cobalt results in a highly active catalyst. This result, however, does not come without cost because rhenium is a relatively expensive commodity.
Thus, there exists a need for means to better disperse the cobalt with a lesser amount of rhenium. Indeed, an object of the present invention is to provide a method for preparing a catalyst having highly-dispersed, small particle-size cobalt, with a homogeneous nanoscale cobalt distribution and which contains 0.2 wt % or less of rhenium.